Nitrogen Isotope Evidence for Redox Variations at the Ediacaran-Cambrian Transition in South China

Abstract

Nitrogen isotope (δ15N) analyses of the early Cambrian (ca. 542–520 Ma) strata in the Yangtze Gorges area, south China, reveal a prominent positive δ15N excursion (≥+8%), concurrent with a negative shift in organic carbon isotopes (δ13Corg) at the early Nemakit-Daldynian or early Fortunian stage. This positive δ15N excursion, in combination with the previously reported paired δ13Ccarb-δ13Corg at this interval, can be interpreted as resulting from denitrification with large isotopic fractionation. Subsequent δ15N from early Nemakit-Daldynian to early Tommotian (stage 2) are dominated by stable values fluctuating between −2‰ and +2‰, close to that of atmosphere N2 (∼0‰) accompanied by the low and stable δ13Corg (∼−33‰) and decoupled δ13Ccarb-δ13Corg. We argue that the relatively low and invariant δ15N values from this interval likely resulted from enhanced N fixation with limited isotopic fractionation. Enhanced N fixation is generally associated with significant nutrient N loss mainly through denitrification and anammox, indicating the expansion of an anoxic environment. The δ15N records of the study sections show large difference from that of potential time-equivalent Zhujiaqing Formation in the Xiaotan section. This discrepancy can be plausibly attributed to different redox conditions in a stratified ocean inherited from the Ediacaran. Anoxic ocean conditions may have lasted for millions of years until the early Atdabanian (stage 3), when trilobites started to dominate the Cambrian ocean.